Abrupt increase of information to be transmitted on an optical communication system has requested an optical transceiver operable in further faster speed exceeding 10 Gbps and sometimes reaching 100 Gbps. In addition to the increase of the operation speed, some optical transceivers implement a function of the coherent modulation where an optical signal is modulated in phase of light. Such a transceiver or the optical communication system to modulate the phase of the light are called as the coherent optical transceiver and the coherent optical system.
In a coherent optical transceiver, an optical modulator types of, what is called, the Mach-Zehnder (MZ) modulator, is inevitable to modulate a phase of light. The MZ modulator is conventionally made of dielectric material, typically a lithium niobate (LiNbO3), because of a large coupling efficiency between electrical properties and optical properties thereof. However, the MZ modulator made of the dielectric material has large dimensions to show an enough interaction, which makes hard to be installed within an optical transceiver with a limited outer dimensions.
Another type of the MZ modulator primarily made of semiconductor material has been developed. Because of larger refractive index of semiconductor materials compared with those of dielectric materials, the MZ modulator made of semiconductor materials has smaller dimensions so as to be installed within a small sized optical transceiver. However, as a compensation of the smaller dimensions, the MZ modulator of semiconductor materials inevitably or inherently shows a greater optical loss. Accordingly, a means to amplify an optical signal output from the MZ modulator, or entering the MZ modulator, that is, an optical amplifier type of erbium doped fiber amplifier (fiber amplifier), is necessary to be installed within the optical transceiver.